A transformer is a device that transfers electrical energy from one circuit to another by magnetic coupling. Typically, a transformer includes one or more windings wrapped around a core. An alternating voltage applied to one winding (a “primary winding”) creates a time-varying magnetic flux in the core, which induces a voltage in the other (“secondary”) winding(s). Varying the relative number of turns of the primary and secondary windings about the core determines the ratio of the input and output voltages of the transformer. For example, a transformer with a turn ratio of 2:1 (primary:secondary) has an input voltage that is two times greater than its output voltage.
A transformer tap is a connection point along a transformer winding that allows the number of turns of the winding to be selected. Thus, a transformer tap enables a transformer to have variable turn ratios. Selection of the turn ratio in use is made via a tap changer switch.
A dual voltage transformer is a transformer that includes two windings, which can be connected in series to handle a specified voltage and amperage, or in parallel to handle double the amperage at one half the series connected voltage. The voltage is changed by operating a dual voltage switch. For simplicity, the term “switch” is used herein to refer to either a tap changer switch or a dual voltage switch.
It is well known in the art to cool high-power transformers using a dielectric fluid, such as a highly-refined mineral oil. The dielectric fluid is stable at high temperatures and has excellent insulating properties for suppressing corona discharge and electric arcing in the transformer. Typically, the transformer includes a tank that is at least partially filled with the dielectric fluid. The dielectric fluid surrounds the transformer core and windings.
A core clamp extends from the core and maintains the relative positions of the core and the windings in the tank. A switch is mounted to a side wall of the tank. The switch includes one or more contacts electrically coupled to at least one of the windings, for altering a voltage of the transformer.
Metallic screws fasten the contacts to a housing of the switch. The contacts and screws are live (i.e., electrically charged). The core clamp and tank wall are electrically grounded. The metallic screws provide decreased electric clearance with the grounded tank wall. The sharp screw points and air trapped in the screw holes also decrease dielectric and radio influence voltage (“RIV”) performance in the transformer.
To meet minimum electrical clearance to ground requirements, there must be at least a minimum distance between the live contacts and screws and the grounded tank wall and core clamp. As the size of the switch (and/or the switch's contacts and/or screws) increases, the tank must get wider or the switch must be mounted above the core clamp, in a taller tank, to meet the minimum distance requirement. As the size of the tank increases, the cost of acquiring and maintaining the transformer increases. For example, a larger transformer requires more space and more tank material. The larger transformer also requires more dielectric fluid to fill the transformer's larger tank. Thus, the cost of the transformer is directly proportional to the size of the switch.
Therefore, a need exists in the art for a switch having a decreased size. In addition, a need exists in the art for a switch with increased electrical clearance with the grounded tank wall and increased dielectric and RIV performance. A further need exists in the art for a switch devoid of metallic screws for fastening the switch contacts to the switch housing. A further need exists in the art for a switch devoid of metallic screws for any purposes.